Fuel control system

ABSTRACT

A fuel control system for an engine, laving a pup operable to supply an output flow of fuel, engine fuel flow regulation means operable to regulate the pump output flow, and pump control means operable to vary an output rate of the pump according to an operational condition of the engine fuel flow regulation means.

This application is a division of application Ser. No. 09/561,124, filedApr. 28, 2000 U.S. Pat. No. 6,412,271.

This invention relates to a fuel control system and a valve therefor.More particularly, but not exclusively, the invention relates to such asystem and valve for use with a gas turbine engine for aerospaceapplications.

The fuel supply system for a gas turbine engine generally comprises ahigh-pressure fuel pump and series metering valve. In order to provideconsistent, accurate, metering of fuel, a spill valve is provided tomaintain a substantially constant pressure drop across the meteringvalve. Where the output flow rate from the pump significantly exceedsthe metered flow requirement, a large volume of fuel is returned by thespill valve to the pump inlet. This high re-circulated volume results inundesirable heat rejection into the fuel.

In accordance with the invention, a fuel control system for an enginecomprises a pump operable to supply an output flow of fuel, engine fuelflow regulation means operable to regulate the pump output flow, andpump control means operable to vary an output rate of the pump accordingto an operational condition of the-engine fuel flow regulation means.

Conveniently, the engine fuel flow regulation means comprises a meteringvalve, and the pump control means is operable to vary the output rate ofthe pump according to the operational condition of the metering valve.

A fuel control system for an engine comprising a pump operable to supplyan output flow of fuel, a metering valve operable to regulate the outputflow, and auxiliary flow control means operable to vary an auxiliaryflow of fuel from the output flow according to an operational conditionof the metering valve.

Additionally, a pressure drop and spill valve may be connected acrossthe metering valve. This facilitates accurate and rapidly responsivecontrol of the regulated flow, which complements the relatively slowcontrol of the output flow provided by the pump control means.

Preferably, the pump control means comprises a servo device for varyingthe output rate of the pump in response to a servo flow, and a servoflow port arrangement integral with the metering valve for controllingthe servo flow to the servo device. Appropriate pump control can beprovided in this manner without the need for a discrete servo valvearrangement, facilitating a reduction of the volume and weight of thefuel control system.

Conveniently, the pump control means is operable such that the pumpoutput rate is increased to a predetermined maximum when a low pressureservo flow is supplied to the servo device and decreased to apredetermined minimum when a high pressure servo flow is supplied to theservo device. This alleviates the potential problem of a depletion ofthe output flow to the engine by a high-pressure servo flow when theengine requires maximum available regulated flow.

The engine fuel flow regulation means may comprise a pressure drop andspill valve operable to regulate a pressure drop across a metering valveby spilling fuel, the control means being operable to vary the outputrate of the pump according to the operational condition of the pressuredrop and spill valve.

The pressure drop and spill valve facilitates accurate and rapid finetunning of the system parameters whilst the pump control meansfacilitates the provision of slightly slower, more approximatecontinuous output flow adjustments to reduce spill volume, with a viewto achieving near optimum pump delivery and heat rejection across a widerange of operating conditions.

Preferably, the pump control means comprises a servo device for varyingthe output rate of the pump in response to a servo flow, and a servoflow port arrangement integral with the pressure drop and spill valvefor controlling the servo flow to the device. Appropriate pump controlcan be provided in this manner without the need for a separate servovalve and/or servo position sensor arrangement, facilitating a reductionin the volume and weight of the fuel control system.

In another aspect, the invention provides a metering valve for a fuelcontrol system or a pressure drop and spill valve for a fuel controlsystem, the valve including an auxiliary flow port arrangement forvarying an auxiliary flow from the valve to a bearing surface of a fuelpump of the system and/or a servo flow port arrangement for controllinga servo flow from the valve to a fuel pump control servo device.

According to a further aspect of the invention, there is provided a fuelcontrol system for an engine, comprising a pump operable to supply anoutput flow of fuel, a valve operable to regulate the output flow, andauxiliary flow control means operable to vary an auxiliary flow of fuelfrom the output flow according to an operational condition of the valve.This permits reduction of the auxiliary flow to minimize depletion ofthe output flow and maximize the regulated flow available to the engine.

The auxiliary flow control means may include an auxiliary flow portarrangement integral with the valve, for varying the auxiliary flow.This removes the need for a separate auxiliary flow control valve and/orsensor, facilitating a reduction in the volume and weight of the fuelcontrol system. The term “engine fuel flow regulation means” as usedherein refers to any means suitable for regulating pump output flow, forexample a metering valve and/or pressure drop and spill valve.

In order that the invention may be well understood, three embodiments ofthe invention will now be described, by way of example only. withreference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing showing a portion of a fuel control systemfor a gas turbine engine;

FIG. 2 is a schematic drawing showing a portion of another fuel controlsystem for a gas turbine engine;

FIG. 3 is a view similar to FIG. 2 showing the system in a differentoperating condition thereof;

FIG. 4 is a view similar to FIG. 2 showing the system in still anotheroperating condition thereof; and

FIG. 5 is a schematic drawing showing a portion of still another fuelcontrol system for a gas turbine engine.

Referring to FIG. 1, a fuel control system 1 includes a variabledisplacement vane pump (shown in cross-section) mounted using bearings2, two of which are shown. The pump is operable to supply an output flowof fuel through a passageway 3 to a flow regulation chamber 4 of anoutput flow-regulating valve 5, commonly referred to as a meteringvalve. The metering valve 5 is operable to regulate the flow of fuelpassing through the chamber 4 through an orifice 6, to supply a desiredregulated flow of fuel along a passageway 7 in the direction of arrow Ato a gas turbine engine (not shown) in accordance with the operatingrequirements of the engine. Flow of fuel through the orifice 6 iscontrolled by movement of a metering valve member 8 relative to theorifice 6 to vary the effective size of the orifice 6. A pressure dropand spill valve (not shown) would normally be connected across themetering valve 5 to maintain a desired pressure across the meteringvalve 5 in a known manner.

The metering valve 5 also has an auxiliary flow chamber 9 in fluidcommunication with the flow regulation chamber 4. In FIG. 1, the chamber9 is shown in fluid communication with an auxiliary flow orifice 10through which fuel is supplied to inner surfaces 11 of the bearings 2,for lubricating the surfaces 11.

In use, low-pressure fuel is fed into the vane pump along a passageway12 in the direction of arrow B. The fuel is discharged from the vanepump into the passageway 3 at high pressure. In this description, theterms low pressure and high pressure are to be understood to relaterespectively to the pressure of fuel prior to entering the vane pump andthe pressure of fuel on the discharge side of the vane pump.

In its position shown in FIG. 1, the valve member 8 provides anintermediate level of regulated flow through the orifice 6 and alubricating flow of high pressure fuel is supplied to the bearingsurfaces 11 through the orifice 10. Such an intermediate flow would beusual, for example, during engine cruise and idle conditions, which inthe exemplary system demand a regulated flow above 600 pph.

During periods of increased engine flow requirement, for example undertake-off and climb conditions, the valve member 8 is moved relative tothe orifice 6 axially upwardly in the orientation shown in FIG. 1, andit will be appreciated that the orifice 10 remains in communication withchamber 9 to thereby continue the high pressure fuel supply to thebearing surfaces 11.

During periods of low engine flow requirement, for example for lowengine speeds under relight and windmill conditions, which in theexemplary system demand a regulated flow below 430 pph, the valve member8 is moved axially downwardly in the orientation shown in FIG. 1 so asto reduce the effective area of the orifice 6. The orifice 10 will thenno longer be in fluid communication with the chamber 9, so that thehigh-pressure fuel supply to the bearing surfaces 11 is interrupted. Thelow engine speed results in a low pump speed, and a consequentrelatively low rate of output flow through the passageway 3. Cutting offthe lubricating fuel supply to the bearing surfaces 11, through whichthere may be a flow rate of about 1070 pph at 90 psid, preventsunnecessary depletion of the output flow, thereby ensuring an adequateregulated flow to the engine. Relight can be effected at 4.8% speedusing the system 1. Because of the low pressure rise across the pump andlow pump speed, cutting off the lubricating flow to the bearing surfaces11 does not have any damaging effect on the bearing surfaces under theseconditions.

The auxiliary flow port arrangement incorporating orifice 10 and chamber9 is provided by the metering valve 5, removing the need for a separateauxiliary flow control valve and sensor which would be required toconfirm that the flow control valve is operating correctly. The controlof auxiliary flow to the bearing surfaces 11 is particularly reliablebecause of the inherent reliability of the metering valve 5 of which theauxiliary flow port arrangement 9,10 forms a part.

FIG. 2 relates to another fuel control system 20 having a metering valve21 operable to regulate the flow of fuel to supply a desired regulatedflow of fuel to an engine (not shown) in the direction of arrow A. Thevalve 21 incorporates an auxiliary flow port arrangement similar to thatdescribed in relation to FIG. 1. FIGS. 3 and 4 relate to the systemshown in FIG. 2, and show the metering valve in various alternativeoperating positions. Features of the system 20 that are similar tofeatures already described with reference to FIG. 1, are given the samereference numerals as in FIG. 1 and to avoid repetition will not bedescribed again in detail.

The metering valve 21 has a valve member 22 and defines a servo flowchamber 23 and a servo flow orifice 24. The orifice 24 is in fluidcommunication via a path 24A with a piston control chamber 25 of a servodevice 26 having a piston 27. The system 20 incorporates a variabledelivery vane pump 28 The delivery capacity of the pump 28 is variableby moving a pump housing 29 about a hinge 30 relative to a pump rotor31, so that the control axis of the housing 29 can be offset by avariable amount from the rotational axis X of the rotor. A connectingrod 32 connects the piston 27 to the housing 29 at the side of thelatter opposite to the hinge 30, to enable the capacity of the pump 28to be varied by actually moving the piston 27.

The position of the valve member 22 in FIG. 2 provides a low regulatedflow in the direction of arrow A. In this operational condition, the lowspeed of the engine results in a low pump speed and a resultantrequirement for a high pump discharge rate relative to the pump speed toprovide sufficient regulated flow. It will be apparent that valve member22 closes orifice 10, cutting off auxiliary lubricating flow to therotor berg surfaces (not shown) and thereby alleviating unnecessarydepletion of the output flow as described in relation to FIG. 1. Closureof orifice 10 also cuts off high pressure supply to chambers 23 and 25via orifice 25A enabling a biasing spring 34 of the servo device 26,assisted by a high pressure flow applied to the right hand side of thepiston 27, to drive the piston 27 to the left in the orientation shownin FIG. 2 and move the pump housing 29 into the maximum dischargeposition of the pump. High flow is thus delivered via the passageway 3,with any excess spilt along passage 3A in the direction of arrow C tolow pressure under the control of a pressure drop and spill valve (notshown).

When an intermediate level of regulated flow is required, valve member22 is moved axially upwardly in the orientation shown so as to increasethe effective area of orifice 6 and place auxiliary flow orifice 10 incommunication with chamber 9, as seen in FIG. 3, to provide highpressure auxiliary flow to the pump rotor bearing surfaces as indicatedby arrow D. Since the auxiliary flow communicates with chamber 23 alongpassageway 35, the piston control chamber 25 is subject to high pressureflow via the passageway 24A causing the piston 27 to move to the rightin the orientation shown and drive the pump housing 29 to apredetermined minimum discharge flow position. Under such intermediateregulated flow conditions, the engine is driving the pump at arelatively high speed, and moving the pump 28 into a minimum dischargerate condition therefore minimizes the amount of spill in the directionof arrow C.

When a high-regulated flow is required, the valve member 22 is movedaxially upwardly in the orientation shown to the position seen in FIG.4. The high engine speed in this operational condition drives the pump28 at high speed, necessitating a lubricating and cooling flow to thepump bearing surfaces 11. It will be apparent that orifice 10communicating with chamber 9 provides the necessary high-pressure flowto the bearing surfaces 11. However, the orifice 24 feeding thepassageway 35 is closed by valve member 22, preventing high pressureflow to the piston control chamber 25 and causing the pump to move intothe maximum discharge position shown in FIG. 4. This enables the pump 28to supply the necessary high rate of regulated flow via the meteringvalve 21 in the direction of arrow A. Spill flow in the direction ofarrow C will be minimal in this condition because of the high engineflow requirement. It will be apparent that, whilst the embodiment shownin FIGS. 2 to 4 is particularly advantageous, the exemplary pump controlmeans 23,24,25,27 could be provided independently, without the auxiliaryflow port arrangement 9,10.

FIG. 5 shows a portion of a further fuel control system 41 comprising ametering valve 42, operable to regulate a flow of fuel to supply adesired regulated flow in the direction A to a gas turbine engine. Apressure drop and spill valve 43 is connected across the metering valve42. The system 41 also includes a variable displacement vane pump 28 anda servo device 26. Features of the system 41, which are identical tofeatures already described with reference to FIGS. 1 and 2 to 4, aregiven identical reference numerals and are not described in detailagain.

The piston control chamber 25 of servo device 26 is supplied from aservo flow port arrangement including an orifice 44 and a servo flowchamber 46 defined by a control element 45 of the pressure drop andspill valve 43. The arrangement is such that the effective area of theorifice 44 is continuously varied in accordance with pressuredifferential across the metering valve 42 by movement of the controlelement 45, thereby continuously varying the high pressure servo flow tothe piston control chamber 25 and consequently the displacement and thedischarge rate of the pump 28. At take-off/climb and relight conditionsthe fuel requirement of the engine approaches the maximum deliverycapacity of the pump so that metering pressure drop tends to decrease.This causes spill control element 45 to move upwards reducing spill flowto a low level, in which position the element 45 cuts off high pressureservo flow to the pump thus providing maximum pump displacement. Fullyvariable control of the pump discharge rate can be provided underintermediate flow conditions when orifice 44 is partially open toachieve near optimum pump delivery and spill flow in accordance with theregulated flow requirement of the engine. If pump delivery exceeds theoptimum value, the spill volume will increase thus spill element 45 willmove downwards further opening the orifice 44 and causing the pumpdischarge to be reduced until the spill element and thus pump dischargereturns to the optimum value.

The general pressure drop and spill valve arrangement shown in FIG. 5 issimilar to that shown in EP-A10107940, which is incorporated herein byreference, at page 5 and FIG. 3. However, other suitable pressure dropand spill valve arrangements may alternatively be employed, as will beapparent to the skilled person.

Control of the pump displacement has a relatively slow effect on theoutput flow and may be inaccurate due to hysteresis in tie pump variabledisplacement mechanism, whereas the pressure drop and spill valvequickly affects the regulated flow and provides accurate fine control,resulting in control loops with different reaction times.

The metering valve 42 can incorporate an auxiliary bearing flowarrangement as described above with reference to FIG. 1.

As described above, the pump control may select only a maximum orminimum pump output flow rate or may be able to control output rate toany value between maximum and minimum. The two position controlarrangement substantially reduces pump delivery and thus heat rejectionto fuel at the critical idle and cruise conditions, while the infinitelyvariable control arrangement seeks to achieve near optimum pump deliveryand heat rejection at all operating conditions at the expense of a morecomplicated control arrangement.

What we claim is:
 1. A fuel control system for an engine, including apump operable to supply an output flow of fuel, engine fuel flowregulation means comprising a metering valve operable to regulate thepump output flow, and pump control means comprising a servo device forvarying the output rate of the pump in response to a servo flow, and aservo flow port arrangement integral with the metering valve forcontrolling the servo flow to the servo device whereby to vary an outputrate of the pump according to an operational condition of said meteringvalve.
 2. A system as claimed in claim 1, wherein the pump control meansis operable such that the pump output rate is increased to apredetermined maximum when a low pressure servo flow is supplied to theservo device and decreased to a predetermined minimum when a highpressure servo flow is supplied to the servo device.
 3. A system asclaimed in claim 1, further comprising auxiliary flow control meansoperable to vary an auxiliary flow of fuel from the pump output flow toa bearing surface of the pump according to the operational condition ofsaid metering valve.
 4. A system as claimed in claim 3, wherein theauxiliary flow control means includes an auxiliary flow port arrangementintegral with said metering valve for varying the auxiliary flow.